Question : External DNS servers will not cache customer MX records with 0 priority

I have two external DNS servers that will not cache daimlerchrysler.com and dcx.com MX records.  What are the typical causes of this?  They cache chrysler.com MX records fine.  The only difference is that the priority is 10 for chrysler.com MX records and 0 for all daimlerchrysler.com and dcx.com MX records.

Answer : External DNS servers will not cache customer MX records with 0 priority

have you checked the TTL for the daimlerchrysler.com and dcx.com zones.  Perhaps they are very short.  I checked dcx.com and the TTL for the MX record appears to only be an hour so the cache would expire every 60 minutes.